We've enough creativity in the side. The problem is that creativity needs a foundation of CM strength before it can flourish, and United have lacked that strength to date.
If you look at the midfields of City, Chelsea and Liverpool in their previous games you see the following CM partnerships:
Toure/Fernandinho
Lampard/Ramires
Gerrard/Lucas
They're not particularly creative players, they're generally workhorses; Gerrard can pick a good pass but no more so than Michael Carrick. Importantly though each partnership contains physically strong players and a goalscorer. It's in front of them where you'll find the creativity in Aguero, Oscar and Aspas. More creativity flanking them in Silva, Hazard and Coutinho.
Compare those partnerships with United's prior to Fellaini. Where's the workhorse? Where's the goalscorer? Where's the physicality and the steel? Creativity doesn't come from the CM position in the Premier League. You need solidity in the center of the pitch; physically strong players who can win the ball, hold it up and give it quickly to those more creative around them. Without this a Kagawa/Rooney type AMC sat in front of the midfield just cannot get on the ball with enough regularity to be effective. How many times last season did Kagawa look impotent or Rooney have to drop deep to find possession? This isn't due to a lack of creativity behind them, but a lack of solidity.